The invention relates to a method for actuating a spark gap in an internal combustion engine, in particular a spark plug, in which the spark gap is assigned a first ignition coil and a second ignition coil, each of which has a primary winding and a secondary winding that are inductively coupled to one another.
EP 2 325 476 A1 discloses a control unit for a spark plug in an internal combustion engine, said unit making it possible to increase the duration of the ignition spark. For this purpose, two ignition coils are assigned to the spark plug and are operated in a manner offset over time (controlled by a control device). The method starts in that a start signal for the ignition of the spark plug comes from an engine control unit, whereupon both primary coils are connected to the vehicle battery or to the dynamo of the vehicle and are charged. This occurs as long as the start signal coming from the engine control unit is present. When it disappears, the two primary windings are discharged by opening semiconductor switches that are arranged in the electrical circuit of the primary windings. As a result, a high voltage is induced in each of the secondary windings, which leads to a discharge between two electrodes of the spark plug. The two semiconductor switches are subsequently opened and closed alternately so that one of the two ignition coils always stores magnetic energy whilst the other delivers the stored energy to the spark plug. If the primary current exceeds a predefined limit value, it is restricted by opening a bypass so that the ignition coils do not reach magnetic saturation. The bypass continues to be opened and closed so as to thus keep constant the energy stored in the ignition coils. The semiconductor switches are switched over whenever the amperage of the secondary current falls below a predefined minimum. This minimum is determined newly in each cycle as a function of the maximum encountered primary current. A diode that blocks the secondary current whilst the primary winding is charged and allows the secondary current to pass whilst the primary winding is discharged is located in the electrical circuit of each secondary winding. To protect the diode against overload, the gradient over time of the secondary current, which is a measure for the magnitude of the secondary voltage, is monitored and is interrupted if a specific voltage level of the ignition process is exceeded. A disadvantage of this prior art is that, in spite of a considerable control effort, it is difficult to create stable conditions at the spark plug for a discharge process lasting for a predefined period of time.